The present invention relates to electromagnetic flow meters. However, aspects of signal processing techniques disclosed herein may be more broadly applied. The operating principles of Electromagnetic Flow Meters are well known, discussed for example in GB-A-2,380,798.
Where the sensing electrodes are in contact with the fluid, due to electrochemical or other effects, a DC potential is usually present across the electrodes even when there is no coil excitation, i.e. no field. That component of the signal is independent of the flow and is generally not static. For example, it may drift randomly with time, flow and temperature. This inhibits the ability to determine the flow in a static fashion. To overcome this some form of dynamic excitation to the coils is typically provided in order to generate a dynamic component at the electrodes that can be differentiated from the background DC (or slowly varying) bias signal. This dynamic signal is normally pulsed DC or an AC signal.
While using some kind of alternating signal to excite the meter is usually necessary, it does introduce its own particular problem, namely that electromagnetic coupling between the coil current and the electrode wires typically creates a signal at the electrodes when there is no fluid flow through the conduit. This signal is completely independent of any flow-generated signal at the electrodes and so the total signal received is the sum of the unwanted ‘zero’ term (the non-flow component) and the flow generated signal.
In a sinusoidally excited meter, this unwanted signal is typically at nominally 90 degrees to the wanted signal and hence is often termed the ‘quadrature’ signal. The unwanted signal is related to the rate of change of current in the coils and consequently is often described as being due to the ‘transformer effect’ where the coil winding is the primary and the electrode wiring is the secondary. In a perfectly symmetrical sensor, the signal created in the electrode wiring in a typical sensor should be zero but manufacturing tolerances mean that there is always some residual area in the electrode wiring ‘loop’ that picks up some of the primary current.
In a sinusoidally excited meter it is known to include some kind of phase adjustment in the phase-sensitive detector to null out this unwanted signal (this process can be performed in hardware, manually or automatically or in software). Performance of the system is often limited by the success of this zero removal system. In many systems where the adjustment is only made at one operating condition (factory calibration) then the system may exhibit errors at other operating conditions, typically temperature or installation related.
AC meters excited with a sinusoidal waveform (or a plurality of frequencies as we have previously shown) can be beneficial but require substantially continuous excitation. To reduce power consumption, for example in a battery-powered meter, intermittent excitation is preferred, and pulsed DC meters are typically used.
In ‘pulse’ or ‘square wave’ driven meters, the approach is usually to provide adequate settling time after the coil current is changed for the transient quadrature signal to die away (since it is related to the rate of change of coil current). These meters work well because there is substantially no quadrature signal but the need for the signal to die away has implications on the maximum speed at which the meter can be operated.
Conversely, if a measurement is not taken sufficiently quickly, there is a potential problem of zero drift. The “square wave” will not normally be true square wave as the slew rate will be limited by the apparatus and will often be deliberately limited to reduce effects of the high rate of change of current.